One breath powers every note
Inside a melodica, each playable note has its own thin metal reed, fixed at one end above a small opening. When air passes the reed, it flexes back and forth rapidly, making the air vibrate and producing a pitch. Pressing a key does not strike anything; it opens a valve that lets air reach the matching reed.
That makes the melodica a free-reed instrument: the reed vibrates freely in its opening rather than being plucked or bowed. The same basic principle is used in instruments such as harmonicas and accordions, though their airflow and key systems are arranged differently.
The player shapes sound with breath
The keys select pitches, while the player’s breath supplies the energy. Blow harder and the notes generally become louder; ease the airflow and they soften. A keyboard player can hold a note without supplying any effort, but a melodica player must sustain the airflow to keep the reed vibrating. Breath therefore becomes part of the phrasing, much as it does for a wind instrument.
Several keys can be pressed together to sound a chord. But all those notes draw on the same stream of air, so the player has to manage one breath across them. The result is a keyboard whose range of notes is familiar, yet whose sound depends on lungs as well as fingers.
